SPHINX: A Study in Convergent Telephony

In this paper, we study protocol convergence and inter-operation using a model called SPHINX (SiP, H.323 and IN inter-working). The intent here is not to compare these protocols with a view to determining where or how one is better than another, but rather to study issues that would enable proponents of any of tjese protocols to more effectively inter-work them. For purposes of this paper, convergence is defined as the capability for seamless inter-operation. We study two types of convergence - one between protocols within the access domain (e.g., H.323 and SIP inter-working), and another along the service-and access-domain interface (e.g., SIP clients accessing IN-supported services). Some basic working knowledge of In, SIP and H.323 is assumed. This paper does not consider all aspects of call control relating to inter-(soft)-switch communication or of the structure of protocols such as BICC (Bearer Independent Call Control), or SIP BCP-T (Best Current Practices for Telephony) that are employed to achieve state synchronization between switching nodes in data networks. These issues form the basis of another paper.